This invention relates to a piston ring for an internal combustion engine and is particularly of the type which, when installed in the groove of an engine piston, assumes a dished (frustoconical) configuration (similar to a Belleville spring) and engages the opposite side walls of the piston groove with portions of opposite radial ring faces under axial tension.
German Offenlegungsschrift (Non-examined Published Application) No. 3,305,873 discloses a piston ring of the above-outlined type. The piston ring engages, in the zone of its radially outer circumference, the upper side wall of the piston groove, that is, that side wall which is closer to the combustion chamber and, along the zone of its radially inner circumference, the piston ring engages the lower side wall of the piston groove, that is, that side wall which is closer to the crankcase. The piston ring engages the side walls of the piston groove along a line of contact and also engages, in the same manner, the base wall of the groove as well as the running face of the cylinder wall. By virtue of this arrangement the piston ring is incapable of displacement either in the axial or in the radial direction relative to the piston. Since the axial height of the cross-sectionally rectangular piston ring is, in its relaxed state, significantly less than that of the associated piston groove, in the installed state of the piston ring an axial tension therein is generated substantially by the twisting of the ring upon stressing in the circumferential direction. This occurrence is analagous to the known "reverse torsion" rings. By virtue of the above-noted linear contact between the piston ring and the piston groove during the operation of the engine, significant wear occurs both at the piston ring and in the corresponding zones of the piston groove.
Japanese Patent No. 136,063 seeks to cure the above-outlined problem by disclosing a piston ring of the above-outlined type which, at least in its zones cooperating with the side walls of the piston groove, has wear-resistant properties. With such an arrangement, to be sure, the problem of piston ring wear may be resolved, but the problem of groove wear persists: in fact, risks are high that the hardened zones of the piston ring will entrench into the side walls of the piston groove even more rapidly than before.